Q400 Vmo restrictions.
Hi all experts!
I just finished my rating on the Q400 and have a question about the "false" Vmo restriction imposed for flying below FL100. Our OM-B states the following limitations:
0 - 8.000ft 245KIAS
@10.000ft 282KIAS
@18.000ft 286KIAS
@20.000ft 275KIAS
@25.000ft 248KIAS
@27.000ft 238KIAS
Naturally, there have been quite a few Vmo exceedances during initial training for some of our crew, as they descend through 10.000ft and the Vmo suddenly drops almost 40 knots the next 2.000ft (which is highly illogical and unusual for any aircraft btw.). We're used to the -300 version, which does not have this problem, and we fly a mixed fleet.
From what I understand, DeHavilland has programmed the Vmo pointer to indicate a fictious limit, in order to prevent pilots from exceeding 250KIAS below FL100. This due to the wind shield not being able to cope with a bird strike at speeds above 250KIAS.
Now, the 250 knots limit below FL100 is an international speed restriction (?), mostly because of the danger of bird strike at altitudes below 10.000ft. But why impose a "false" restriction instead of just trusting the pilots' judgement on this? Other aircraft do nor have this limit built into their Vmo pointers. The "free speed" option is often given by ATC. I consider it acceptable to keep, say 300KIAS, below FL100 if the risk of bird strike is negligible/unlikely; e.g. birds don't fly at night or in clouds, so why the need for speed reduction?
Is it an FAA/JAA restriction, commonality issue with the DHC8-100/-200/-300, or something else?